In cellular (mobile) telecommunications networks, the ability of a communications node to communicate over a downlink control channel with each UE within its cell is determined by the signal-to-noise ratio on the channel which is related to the range of the UE from the base station.
In order to address this issue, it has been proposed for 3GPP to have multiple Modulation and Coding Schemes (MCS) for the layer L1-layer L2 control structure with the MCS selected by the communications node for a downlink control channel to UE being determined by the range of the UE from the communications node at that time. In this particular proposal, the communications node cell is divided into a number of communication regions of successively greater range from the communications node and each communication region is allocated a particular Modulation and Coding Scheme so that the MCS selected for a particular UE will depend upon the communications region within which the UE is situated at that particular time. FIG. 4 of the accompanying drawings illustrates the MCSs to be employed for the Long Term Evolution (LTE) of UTRAN. In this example, four communications regions are provided defined by successive concentric circles. As proposed, all of the MCSs use the same modulation type, QPSK (Quadrature Phase Shift Key), but have different coding rates. In the example shown in FIG. 4, the innermost or first communications region has a coding rate of 2/3, the next or second communications region has a coding rate of 1/3, the third communications region has a coding rate of 1/6 and the fourth or outermost communications region has a coding rate of 1/12. A communications node may thus apply adaptive coding for the DL control channel for a UE, depending upon the communications region in which UE is present at any given time.
The transmission bandwidth available for control channels is divided into a number of control channel elements. A control channel consists of one or more logical control channel elements (CCEs) where a CCE consists of a number, for example 12, of resource elements or sub-carriers. Thus, the smallest control channel consists of a single CCE and larger control channels can be formed by aggregating multiple CCEs.
The downlink (DL) and uplink (UL) control channels may thus be formed from one, two, three, four or more CCEs, depending on the required payload for the control channel. The coding rate of the MCS affects the payload and thus the required number of aggregated control channels. Where multiple MCSs are used, the number of control channel elements aggregated to form the control channel will differ from communications region to communications region. Accordingly, UE may have to search all the different aggregate combinations to find its control channel from the base station.